This invention relates generally to rotary machines and more particularly, to diaphragm patch rings for use in a rotary machine.
At least some steam turbines have a defined steam path which includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. Many of these steam turbines include stationary nozzle segments that direct a flow of steam towards rotating buckets, or turbine blades, that are coupled to a rotatable member. The nozzle airfoil construction is typically called a diaphragm assembly. Each diaphragm assembly is usually referred to as a stage and most steam turbines have a configuration that includes a plurality of diaphragm assembly stages.
Steam leakage, either out of the steam path or into the steam path, from an area of higher pressure to an area of lower pressure may adversely affect an operating efficiency of the turbine. For example, steam-path leakage in the turbine between a rotating rotor shaft of the turbine and a circumferentially surrounding turbine casing may lower the efficiency of the turbine. Additionally, steam-path leakage between a shell and the portion of the casing extending between adjacent turbines may reduce the operating efficiency of the steam turbine and over time, may lead to increased fuel costs.
In addition to facilitating steam flow, to facilitate minimizing steam-path leakage as described above, at least some known steam turbines use a plurality of labyrinth seals that are integral to the diaphragm assemblies. The seals are typically ring segments that are inserted into circumferential grooves at the radially innermost section of the diaphragm assembly, often referred to as a bore. Some known labyrinth seals include longitudinally spaced rows of labyrinth seal teeth which are used to seal against pressure differentials that may be present in the steam turbine.
Some steam turbine maintenance activities periodically include reducing the associated rotor diameters for a variety of reasons that include accommodating new features such as longer buckets, enhancing rotor stability, and/or mitigating rotor thrust values. In some of these instances, it is desirable to retain and reuse the existing diaphragm assemblies. In the event that the aforementioned seals alone cannot be modified to accommodate the extended gap between the diaphragm assemblies and the rotor, the existing diaphragm may be modified such that the bore of the diaphragm assembly and associated seals can mate with the reduced rotor diameter. In those steam turbine configurations where sufficient radial space exists, welding a diaphragm extension to existing diaphragms may suffice. Furthermore, alternative methods of extension attachment may be considered, such as for example, coupling extensions to existing diaphragms with a dowel-type configuration. However, in some known steam turbines, sufficient space for the aforementioned welding and dowel configurations may not be present and a low-profile, self-supporting configuration may be a solution.